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The final space shuttle flight will now take place in 2011, Nasa has confirmed.

The Endeavour orbiter is targeting a 26 February lift-off for a mission that will deliver a particle physics experiment to the space station.

The agency had hoped to get Endeavour away before the end of this year but the programme is running late because critical payloads are not ready.

The penultimate launch, of Discovery, has also been bumped from September to 1 November.

Discovery, which carries the designation STS-133, is waiting for an upgrade to be carried out on the Italian-built logistics module known as Leonardo.

The module, which is used as a packing box for supplies in the orbiter's payload bay, would normally return to Earth with every shuttle mission, but for Discovery's flight it will be left on station to provide extra storage space.

This requires Leonardo to be fitted with extra shielding to protect it from small meteoritic impacts, and this work cannot now be finished in time to make the September launch window.

The physics experiment to be carried up by Endeavour (STS-134) is also delayed. The Alpha Magnetic Spectrometer, which will search for evidence of dark matter, is still in Europe where a last-minute change is being made to a key component.

Nasa still has to decide if it wants to fly out the launch-on-need shuttle which will be prepared as a rescue ship for Endeavour's last flight. Should Endeavour's crew find their ship is so badly damaged after launch that they cannot return to Earth, the rescue shuttle would be sent up to bring the astronauts down.

The Atlantis orbiter is being prepared for the role, and the US space agency has not excluded the possibility that it could simply fly out this standby shuttle anyway to take additional spares and supplies to the station.

The slip in the shuttle schedule is not unexpected. Historical flight rates suggested that Nasa would do well to complete the Endeavour mission by January.

US President Barack Obama had put additional monies in the Federal Year 2011 budget request to cover such delays.